(1) Field of the Invention
The invention relates to vesicular imaging and to elements used therein. In particular, it relates to novel vesicular imaging elements which include an overcoat, and methods of making and using the element.
(2) State of the Prior Art
The first commercial vesicular photographic materials utilized gelatin binders. Gelatin was replaced with improved binder materials due to the undesired tendency of gelatin to absorb moisture and therefore to release the image-forming bubbles, destroying the image. Typical improved materials included a great variety of synthetic polymers, such as poly(vinyl chloride), poly(vinylidene chloride), and polystyrene; and copolymers obtained by copolymerizing acrylonitrile with vinyl chloride, styrene, vinylidene chlorofluoride, or 1,1-difluoroethylene; by copolymerizing vinyl chloride with methyl acrylate, acrylic acid, diethyl maleate, or vinyl acetate; or by copolymerizing vinylidene chloride with vinyl chloride, vinyl acetate, vinyl alcohol, ethyl acrylate, or acrylonitrile. Examples of the homo- or co-polymers of vinylidene chloride are described in U.S. Pat. No. 3,032,414, issued to R. James.
To be suitable, a binder must be sparingly permeable, must have the proper diffusivity for the light-generated gas, and must have the proper rigidity for generating the image-forming vesicles. Of these three characteristics, permeability and diffusivity are approximately proportional, and even rigidity is related to permeability, as is noted in the aforesaid U.S. Pat. No. 3,032,414. Thus, it is generally sufficient to evaluate only the permeability constant (PC) of a material to determine if the material is satisfactory as a binder. This constant is most readily expressed as PC, equal to the number of cubic centimeters of gas transmitted by 1 square centimeter of the binder material during one second at constant temperature, when the pressure gradient is one centimeter of mercury per centimeter of the thickness of the binder layer, or cm.sup.3 cm.sup.-2 sec.sup.-1 (cm Hg/cm).sup.-1. One method of expressing the range of useful permeability has been that the binder should have a PC of between about 1 .times. 10.sup.-11 as PC.sub.max, and about 1 .times. 10.sup.-15 as PC.sub.min, at a temperature of 30.degree. C. See, for example, U.S. Pat. No. 3,355,295 to Priest.
The most commonly used vesiculating agents are diazonium salts of various kinds. Upon exposure to activating radiation these salts release nitrogen gas. Because of the permeability constant of the binder as described above, the gas is retained within the element until development by heat causes expansion of the gas into light-scattering vesicles. Typical examples of such diazonium salts are taught in U.S. Pat. No. 3,355,295 and in Kosar, Light-Sensitive Systems, (1965), page 277.
It has been recognized in a number of instances that the characteristic high contrast and relatively low speed of vesicular elements need modification. The high contrast is undesirable as it renders it difficult to reproduce a "gray scale". To solve these problems, other vesiculating agents such as azides have been used as in British Pat. No. 956,336 or French Pat. No. 1,281,905, but such other vesiculating agents tend to lack the stability of diazonium salts.
Still other approaches to removing the above-noted problems have involved the addition of other components to the element, such as a dispersible water-soluble organic colloid as in U.S. Pat. No. 2,703,756, or the use of special processing techniques such as a heat treatment, shown for example in U.S. Pat. No. 3,149,971. However, such composition or process additives have introduced an added cost element.
Overcoating has also been tried as a technique to reduce the contrast and increase the speed, as shown for example in U.S. Pat. No. 3,615,475 and U.S. Pat. No. 3,143,418. However, these techniques have certain drawbacks. For example, the overcoat layer of the U.S. Pat. No. 3,615,475 patent includes its own vesiculating agent, and an extension of sensitometric characteristics is provided by requiring that each binder have a different permeability constant so that the exposure of both layers to the same image will produce different densities in each layer. Of course, the fact that a vesiculating agent must be used in both layers introduces an additional expense, and the binders or polymers used are restricted in that significantly different permeability constants are required.
Furthermore, in the teachings of both U.S. Pat. No. 3,143,418 and U.S. Pat. No. 3,615,475, there is no appreciation that when the element is coated the solubility of the overcoat should be different from that of the vesiculating layer. Instead, in the U.S. Pat. No. 3,143,418 patent, for example, the overcoat and the vesiculating layer polymers both can be, e.g., poly(vinyl alcohol). It has been found that the use of the same polymers creates coating difficulties due to "strike-through" of the solvent.
Vesicular elements have also been overcoated for other purposes, for example, using as the overcoat layer a silver-salt emulsion which can be thermographically developed, such layer thereafter being used to subsequently imagewise expose the vesicular layer to ultraviolet radiation. Typical of patents disclosing a silver-salt emulsion overcoated onto a vesiculating layer is U.S. Pat. No. 3,515,547. By virtue of the double exposure and development required in such techniques, they cannot provide an imaging element which relies solely upon the vesiculating agent as the image-providing means.
Diazotype photosensitive materials have been overcoated with polymeric layers to transparentize the photosensitive layer, or to increase the sensitivity of the diazo salt. Such materials are not vesicular elements, and no attempt is made in such cases to insure that the polymeric binder of the diazo salt is substantially insoluble in the solvents used for the polymer of the overcoat. U.S. Pat. Nos. 3,370,949 and 3,353,984 are representative of such disclosures, the latter patent providing for the same polymeric binder to be used in both coatings.
Yet other instances of overcoats applied to a vesicular imaging element are those in which the element, after exposure and development, is coated with a hydrophobic substance to give added water resistance, as shown, for example, in U.S. Pat. No. 2,908,572, or wherein a water-swellable layer is added to permit transfer of the imaging layer, as shown for example in British Pat. No. 1,330,537. Of course, overcoats added after development are incapable of increasing the speed of the element, and transfer layers have not been recognized as solving speed or contrast problems. Furthermore, the transfer layer of the British patent is soluble in the solvent used with the vesiculating layer binder.
Protective overcoats have been applied to photothermographic elements other than vesicular elements, as disclosed for example in U.S. Pat. No. 3,856,526. However, the purpose of such overcoats therein was to provide mechanical protection, and in no way did such overcoats increase the speed and decrease the contrast of the actual image.
Patents relating only to the general background of photothermographic elements include U.S. Pat. Nos. 3,383,213; 2,699,392; 3,620,743; 3,622,335; 3,622,336, 3,933,508; and British Pat. No. 645,825.